ipmi_exporter - use maintainers' image

[holmesb]

Hi,

Probably didn't exist when the chart was created, but Prometheus Community now provide a docker image each time they release. Be good to switch to these, also because the image the chart uses is quite old now (1.2.1 vs 1.6.1).

Cheers.

[kfox1111]

Yeah. looks like a pretty new image. yeah, we should switch to that. We could probably consider contributing the chart upstream too if anyone wants it.